1. Field of the Invention
This invention relates to the field of high frequency filters.
2. Background Art
A high frequency continuous time integrated circuit bandpass filter is comprised of a number of filter sections. These filter sections are, in effect, "RC" circuits in which the resistance (R) and the capacitance (C) of the filter element affect the timing accuracy of the filter itself. In the prior art, these filters have been constructed of discrete elements where each element has an accuracy within a desired range. When implementing a filter in an integrated circuit, however, variations in resistance and capacitance can occur. Thus, the accuracy of an integrated circuit filter varies with the R and C of the integrated circuit. It is desired to compensate for these variations to provide an accurate, predictable, stable filter.
One prior art attempt to compensate for variations in the resistance and capacitance of filter elements is to use a "master/slave" approach employing a reference frequency generator. The transconductance (gm) and phase shift of the "slave" filter sections is controlled by a "master" filter or oscillator referenced to an accurate frequency F.sub.R. Total filter accuracy is maintained by continuously applying the frequency reference F.sub.R. A disadvantage of these prior art filters is the fact that the reference frequency F.sub.R is a noise source that can affect the signal that is being filtered. Such prior art filters are described in "Gyrator Video Filter IC with Automatic Tuning," Moulding, K. W. et al., IEEE JSCC, Vol. SC-15 No. 6, December 1980, and "Design and Performance of a Fully Integrated Bipolar 10.7 MHz Analog Bandpass Filter," Chi-Fa, Chiou, and Rolf Schaumann, IEEE JSCC, Vol. SC-21, February 1986.
Other prior art high frequency continuous time IC filters do not use a "master/slave" approach using a reference frequency. In these other prior art filters, a filter's accuracy is established by trimming an external resistor. Such a filter is described in "Single-Chip Y/C Signal-Processing LSI for 8 mm VCR System," Yamaguchi et al., IEEE BCTM, 1987. This technique does not provide a solution for variations in capacitance which can affect filter performance.
Therefore, it is an object of the present invention to provide a high frequency continuous time filter that does not rely on an applied reference frequency during operation.
It is yet another object of the present invention to provide a high frequency continuous time filter that maintains its accuracy over temperature changes.
It is still another object of the present invention to provide a high frequency continuous time filter that maintains its accuracy during power supply changes.